The present invention relates to an ultraviolet rays emitter for emitting ultraviolet rays on an ink of an ultraviolet curing type which is adhered onto a recording medium, and more particularly to an improvement for implementing a quick curing treatment for the ink of the ultraviolet curing type which is adhered onto the recording medium without giving a thermal damage to the recording medium.
The ink of the ultraviolet curing type is different from ordinary water-based and oil-based inks in that quick curing is carried out and stable printing quality can be maintained without the influence of the physical properties of a recording medium (for example, a print paper) such as an ink permeability if the ink of the violet curing type is adhered onto the recording medium and an ultraviolet ray is then irradiated in a proper amount.
In an ink jet printer using the ink of the ultraviolet curing type, it is necessary to provide an ultraviolet rays emitter for irradiating an ultraviolet ray on an ink adhered onto a recording medium around a recording head for ejecting the ink of the ultraviolet curing type as an ink droplet to be a particle and adhering the same ink onto the recording medium.
There have been proposed various associated ultraviolet rays emitters in which an ultraviolet lamp such as a mercury lamp or a metal halide lamp is served as a light source for emitting ultraviolet rays (for example, see Patent Document 1).
However, a light emitted from the ultraviolet lamp has a continuous spectrum within a wide wavelength region and includes a visible light and an infrared light in addition to a plurality of ultraviolet lights having different wavelength regions. Therefore, there is a problem in that the infrared light in the continuous spectrum gives a thermal damage to the recording medium.
There is a problem in that the structure of a device is complicated and a cost is increased if a band-pass filter is provided in order to remove a hazardous infrared light.
The ultraviolet lamp greatly consumes power. Therefore, there is a problem in that the energy saving of the ultraviolet rays emitter is hard to perform.
In addition, the ultraviolet lamp itself is large. For this reason, there is also a problem in that it is hard to reduce the size and weight of a device model.
For this reason, recently, there has been studied an ultraviolet rays emitter in which a semiconductor light emitting element such as an ultraviolet rays emitting diode (an ultraviolet LED) including no infrared light and capable of emitting only an ultraviolet light in a specific wavelength region and reducing consumed power is employed as a light source, thereby preventing a thermal damage from being given to a recording medium due to content of the infrared light, while saving energy and reducing a size and a weight (for example, see Patent Document 2).
[Patent Document 1] JP-A-2004-1326
[Patent Document 2] JP-A-2003-326691
As compared with the ultraviolet lamp, however, the semiconductor light emitting element has lower irradiation intensity of ultraviolet rays to be emitted. For this reason, an irradiation is to be carried out for a long time before the ink of the ultraviolet curing type, which is adhered onto the recording medium is completely cured. For example, there is a problem in that a print processing speed in an ink jet printer is hindered from being enhanced.
Moreover, a color ink of the ultraviolet curing type contains a coloring material for each color (a pigment or a dye). For example, a white ink of the ultraviolet curing type contains titanium dioxide as a white coloring material. The titanium dioxide has a physical property for absorbing ultraviolet rays having a wavelength of 200 to 375 nm.
If the wavelength region of ultraviolet rays emitted from the semiconductor light emitting element is overlapped with an absorption wavelength region of the coloring material such as titanium dioxide or the like contained in the ink, therefore, the ultraviolet rays irradiated from the semiconductor light emitting element is consumed by an absorption through the coloring material such as titanium dioxide or the like. Consequently, the effect of curing the ink is reduced to half, and furthermore, an irradiation for a long time is required.
Accordingly, it is desirable that a semiconductor light emitting element having a different wavelength region from the absorption wavelength of an ultraviolet absorbing substance such as the coloring material, for example, titanium dioxide should be served as a light source. In case of the semiconductor light emitting element, however, a wavelength range which can be irradiated is restricted to a small range. For this reason, actually, it is hard to maintain sufficient ultraviolet irradiation intensity in a wavelength region in which the absorption wavelengths of various ultraviolet absorbing substances are avoided.
In general, moreover, an ultraviolet ray in a low wavelength region has high energy and can cure, in a short time, the surface of ink that is irradiated. On the other hand, transmission force into the inner port of an ink film is low. For this reason, the ultraviolet curing in the inner part of the ink film tends to be delayed. To the contrary, ultraviolet rays in a high wavelength region have high transmission force into the inner part of the ink film. Therefore, the same ultraviolet rays are effective for the ultraviolet curing in the inner part of the ink film. However, there is a problem in that energy is low, resulting in an increase in a time required for curing the ink.
More specifically, in the case in which a semiconductor light emitting element having a low peak wavelength is used as a light source for emitting ultraviolet rays, the curing can be efficiently carried out in a short time if the adhering thickness of the ink of the ultraviolet curing type is small, and the ultraviolet curing in the inner part is delayed if the adhering thickness is great. For this reason, there is a problem in that stable curing cannot be obtained. To the contrary, in the case in which a semiconductor light emitting element having a high peak wavelength is used, the ultraviolet rays reach the inner part and the curing can be carried out stably even if the adhering thickness of the ink of the ultraviolet curing type is great. However, a long time is required for the curing. As a result, there is a problem in that the curing treatment is delayed.